As a general method, induction surface hardening can produce improved properties as compared to surface hardening by other means such as carburizing, cyaniding and nitriding methods which have been used for hardening of such diverse products as crankshafts, bearing races, drive shafts, axle shafts, steering knuckles, and many other products. Among the improved properties are improved torsional fatigue strength, increased load carrying capacity, increased fatigue endurance life, ability to substitute medium carbon steel for low alloy steel, less distortion, and a surface more easily plated and less susceptible to the effects of molecular hydrogen diffusion or hydrogen embrittlement as a result of the cleaning or plating process. Induction hardening as a manufacturing process and engineering design tool has many desirable attributes. No contact is required between the work load and the heat source. The workpiece may be rapidly heated. Process control variables are simply controlled. Heating efficiency is generally higher than that offered by other furnace type methods. Very high temperatures can be achieved. The heating means is readily adapted to other production processes and methods. Working conditions are relatively cool and clean. Controlled atmospheres or vacuum can be used to protect the workpiece. A particular feature is that in many applications induction heating lends itself to flexible but reasonably close control of the locus of heating so as to restrict heat to localized areas and to desired depths of penetration.
Induction surface hardening of workpieces such as thread-forming screws and bolts and the like is intended to harden the workpiece at preselected areas and to preselected depths, often to just below the thread root, leaving the balance of the screw thread and the core material of the screw in a ductile condition. The avoidance of hardening at unwanted areas and depths is often as important as the accomplishment of hardening at desired areas and depths. For example, it may be desired to harden the leading threads of a screw for proper cutting action or thread forming action while avoiding hardening of following threads so that they remain ductile for best strength and best anchoring and holding action. Where threads are to be hardened, it is often desirable that the bulk of the metal below the threads remain ductile so that a tough strong core for the screw is provided. Obviously, it is desirable that symmetry of hardening within the piece and uniformity of hardening from piece to piece be achieved to the greatest practical extent. For example, to the extent that there is lack of symmetry in hardening around the circumference of a screw section, then at certain circumferential locations either the core will be correspondingly weakened by unwanted hardening or the threads will be defective by reason of insufficient hardening.
The advantages outlined for workpieces that function as thread forming screws, bolts or taps have equal importance in applications in which metal is removed such as is the case with taps, reamers, key seat cutters, milling cutters and various similar cutting action tools. The efficiency, life and precision of operation can be enhanced and improved by the application of the principles of induction surface hardening.
The ability of induction surface hardening techniques to develop the full hardness capabilities of the tool material at the cutting surface and yet retain the desirable tool characteristics of toughness and ductility, and the avoidance of brittleness are important applications of the invention to be described.